Antenna arrays may be used in communications systems that transmit and/or receive radio frequency (RF) signals. Antenna arrays typically include a number of antennas that are spatially separated and may be employed in a number of different wireless applications including radio communications systems, cellular systems, television broadcasting, point to point systems, paging systems, medical applications or the like.
The use of antenna arrays in such systems often provide antenna performance improvements over the use of a single element antenna. These antenna performance improvements for received signals may include improved signal to noise ratio and interference rejection. Antenna performance improvements for transmitted signals may include improved directionality and hence less power towards other co-channel users, security, and reduced transmit power requirements. Antenna arrays may be used for signal reception only, for signal transmission, or for both signal reception and transmission.
A typical application of antenna array systems is in a wireless communication system. Examples include a cellular communication system and a wireless local loop system. Such wireless communications systems typically include one or more communications stations, generally called base stations, each communicating with its subscriber units, also called remote terminals, remote devices, and handsets. In cellular systems, the handset is typically remote, while in wireless local loop systems, the handset is typically in a fixed location.
The antenna array is typically at the base station, but may also be employed at a user terminal. Communication from the remote station to the base station is typically called the uplink and communication from the base station to the remote station is typically called the downlink. In time division duplex (TDD) systems, uplink and downlink communications with a particular remote terminal occur at the same frequency, but during different time slots. In frequency division duplex (FDD) systems, uplink and downlink communications with a particular remote terminal occur at different frequencies and may or may not occur at the same time.
Another factor that impacts wireless communication system performance is the presence of multiple remote terminals in the same environment utilizing the same frequency or channel at the same time. As the number of separate sources or remote terminals utilizing the same frequency increases, the amount of interference in the communications channel tends also to increase.